The core part of a helicopter is its main rotor. With its two and more rotor blades, which are hinged on a rotor head and pivotally supported about their longitudinal axis, the main rotor supplies not only lift but also propulsion.
One or more gas turbines drive the main rotor by a gear unit and a main rotor shaft which is rotatably mounted in the gear unit and secured to the rotor head.
The helicopter often also has a driven tail rotor so that the gear unit has a plurality of power inputs for the gas turbines and several outputs for the rotors.
High standards of safety apply to helicopters. Therefore, the parts and units must be subjected to long tests in the development and trial with an input power of more than 1,000 kW for the main rotor and speeds of approximately 20,000 r.p.m. on the power inputs of the gear unit, this requires a high power test rig installation.
To test power-transmitting parts and units such as gear units, clutches, shafts, etc., two kinds of test rigs are known ("Antriebstechnik" 11, 1972, No. 9, pages 332-336; "VDI-Z" 115, 1973, No. 2, pages 115-121; "Antriebstechnik" 22, 1983, No. 10, pages 32, 34, 36 and 38), so-called brake rigs in which the input power passes via a specimen from a prime mover to a braking device and so-called stress test rigs in which the testing power revolves in a stress circuit in which the specimen is enclosed and a prime mover covers only the power losses resulting therefrom. In the second case, the specimen is loaded by a torque which is composed of the stress torque and the torques to be applied by the prime mover which result from the lost power and the drag torques.
Because of the small energy they require, stress test rigs are especially appropriate for gear units with great transmissible power, for example, in the vehicle and aircraft technology.
The stress torque can be rigidly set by a stress device. But it is also known (DE-CE 43 25 403) to change, control, or regulate the stress torque during the test. For this purpose, an electric control unit regulated by microprocessor serves, for example, an engine or generator, which via a highly geared superimposed gear, such as in the form of a maintained-wave gear unit, engages the stress circuit and produces a turning angle between the input and output. At the same time, the speed on the gear input or gear output can be adapted to the reduction ratio of the specimen without changing the test rig gear unit, since the control unit on the superimposed gear unit sets a corresponding drag speed. It is thus possible upon the same test rig to test gear units of different speed and torque requirements insofar as the input and output shafts of the gear unit are similarly arranged. If that is not the case, the test rigs must be adapted to the spatial conditions of the specimens. This involves a high expenditure when test rig gear units and superimposed gear units have to be exchanged.
The extensive assembly work for adapting the test rig to a new kind of gear unit in addition causes a long down time during which the costly investment cannot be used. Besides, the down times prolong, for example, the development time of the gear unit to be tested.